Ok I follow the RV nomads who routinely run Lithium batteries in parallel. Some up to 7 batteries, all with protection ckts.(bims)
They do not require caps or other parts to connect to an electronic device. There are no voltage spikes. Something like illustrated in the pic would work but when playing with electricity, caution is always the word for your actions.
Of course in the case of those Lithium batteries, the batteries are not disconnected to charge them, as the solar panels are connected through the regulator 24/7 and they are charged during Sun hours.
When two batteries are connected in parallel, the voltages match and the current sent to the motor from each battery is half what one single battery would have to supply. The batteries should be close to the same age and condition or a good battery would end up feeding(charging) a battery that is not in as good condition.
If you really want to get fancy, you can add a marine battery switch which allows you to put battery 1 on, battery 2 on, both batteries 1 and 2 on, or both batteries off. weight would be negligable compared to an additional batteries weight.
A thought here about the length of the wire/cable. Distance Does make a difference as resistance builds up in long lengths of cable, but 3 feet would not have that effect other than having a lot of extra cable that you have to tie to the frame to keep out of the prop.
But the wire/cable size means everything. If you size it wrong so you have just enough capacity, then the resistance will still affect your power flow, the wire/cable can heat up, and you won’t have full power from the motor because you lose power generating the heat in the wire/cable. If the cable is too small, the wire/cable will get hot and if it gets hot enough it will fail.
I would advise just get the same size wire/cable that connects to your battery now. that will be ample size for an additional battery.